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The Optical Magic Lantern Journal and Photographic Enlarger.
105
is by this means that we are enabled toascertain in some measure whether any of them are likely to be wasted.
In the optical lantern there are two lenses, or to be more exact two sets of lenses, the condenser and the objective. The work of the condenser is to collect as much of the light as possible and direct it through the lantern slide on to the objective, which will form an image on the screen.
The diagram, Fig, I., wi!l give an idea how this is effected.
It will immediately occur to some, why have two or four lenses since oue will bend the rays in the direction required if it be of the proper thickness.
Lantern lenses are made in inore than one thickness for several reasons, on account of chromatic aberration, this arises from the fact that light is composed of seyen colours, viz :— violet, indigo, blue, green, yellow, orange and red; these cannot be made to refract at the same angle by any one lens. This produces a fussy appearance of the picture on the screen, and the various colours are to be seen near the margins. Chromatic aberration formerly caused great trouble, as it was apparent in all optical instruments, more especially telescopes and microscopes, until in 1825 Mr. Dollond discovered that by using two lenses of different refractive powers, such as those made from crown and flint glass tespectively, placing them together, it was possible to produce a compound lens which would not show the prismatic colours, or in other words correct the chromatic aberration. This division of light is not an unmixed evil, as the rays represented by the colours have two distinct uses. The violet, indigo, blue, and green give little or no heat, but they act chemically upon many substances, particularly upon the salts of the metals. The other rays, yellow, orange and red, act exactly the reverse way; but for this we should have no photography as now practised, and consequently few lantern slides.
Spherical aberration arises from the inability of the lens makers to produce a lens which will bring all rays to a particular point desired, and this difficulty increases proportionally with the thickness of the lens or the measure of its refractive power. Theeffect of this is tomake the
picture appear indistinct, either in the centre or at the edges, andsometimes allover. Lanternists. who are also photographers will probably ask, why not cut off the rays which cannot be brought to a focus, as with photographic lenses, by using a diaphragm or stop? Such would mean a serious loss of light which cannot well be spared. This cutting off of the outer rays, or stopping down as it is termed, is of little consequence when taking a photograph, as it can be. compensated for by lengthening the exposure. There is one other kind ofaberration or distortion, known as curvilinear distortion, it is present in all single or achromatic lenses, and causes the lines near the margin of the picture to be curved. All these defects can be rectified by adding another lens, except in the case when the combined focus is very short, say three or four inches.
Lantern lenses have improved very much during the last ten years, and we may shortly expect even this difficulty to be entirely overcome by the makers.
Condensers must be made of twoor three lenses on account of the heat produced by the illuminant, which would certainly break the. glass were it abnormally thick.
Lenses are not made haphazard, 7.e., a workman is not given a piece of glass and told to. make it into a lens of a certain focus. There are: mathematicians who can design the curves of a. lens with a precision that will be correct to two. or three decimal places, or within =,4,, part of an inch, and the mechanic can with his tools grind to this degree of accuracy ; but neither he nor the mathematician can tell exactly what the refractive powers of a lens, or rather of the glass may be, until the work is finished, and should it not produce the result expected it must be placed. among the cheaper qualities or re-ground and. re-polished at an expense equal to or perhaps exceeding the cost of the original work. This,. apart from faulty design or bad workmanship, is really the difference between a good lens and a bad or indifferent one.
From what has been stated one would suppose: it an easy matter to decide upon which is the. best form for a condenser, but scarcely any two makers agree on this point.
The patterns in common use are :—(a) Double: plano-convex. (B) Meniscus or moonshape and plano-convex. (c) Meniscus and_bi-convex known as the ‘“ Herschel.” (pd) Plano-convex and crossed lens (Dalmeyer’s pattern). (£) Triple condenser.
The curves are exaggerated to make the distinction clearer, and the place for the light is supposed to be in each case on the right.